Solid state imaging devices have become valuable tools in many optical sensing systems. Charge coupled devices (CCD), charge injection devices (CID) and charge transfer devices (CTD) all have been advantageously used in optical sensing arrays. However, prior solid state imaging devices have been plagued by blooming and have generally not provided adequate electronic aperture control or gamma control when used with video camera systems. Moreover, difficulties have arisen when resetting CIDs when used as imagers.
Blooming is an unpleasant phenomenon which has long plagued designers and manufacturers of solid-state imaging devices. Blooming is defined as a signal charge overflow from brightly illuminated cells in an optical array into neighboring cells which are not as highly illuminated, and therefore results in false signal levels in those cells. When a video signal from a solid-state imager is displayed on a monitor, blooming will cause distortion of the image, and in the case of large overflow, a complete saturation of the picture. Blooming viewed on a monitor can take many forms, depending on the particular device structure and technology used to fabricate the imaging device. In buried-channel CCD technology, for example, blooming is particularly persistent, and takes the form of streaks or charge spreading, particularly along the channels of charge transfer.
A number of techniques have heretofore been devised to minimize or eliminate the blooming phenomenon in buried-channel devices. A typical approach has been to incorporate an overflow drain next to each charge collecting element and drain the overflow charge out in a lateral direction. This approach, even though effective in removing charge overload, consumes focal plane area and has been difficult to fabricate.
Another method of anti-blooming utilizes a buried drain which is located beneath the charge storing elements. In this architecture, the overflow charge is drained from the imaging cell in the vertical direction. This method results in a low quantum efficiency in the longer wavelength spectral region, since the majority of the signal charge resulting from such light is generated beneath the buried drain and is thus lost from collection by the charge storage elements.
There have also been previous proposals using various carrier recombination schemes for elimination of undesirable signals. These have been based on recombination in the bulk on oxide precipitates. However, these techniques have created complications in device processing, as well as difficulties with effective implementation for area sensor blooming protection.
A technique for preventing blooming has been proposed which applies alternating voltage to a gate adjacent to the charge storage region of a charge transfer device to put the surface of the semiconductor substrate into an accumulation or depletion state. Electrons trapped at the surface states are recombined with majority carriers in the accumulation state and therefore excess signal charges are trapped at the surface states vacant by the previous recombination, thus resulting in the removal of excess charges. This technique is described in U.S. Pat. No. 4,328,432 by Yamazaki, issued May 4, 1982. However, this technique has not been useful on all types of solid state imaging devices and has not been heretofore useful in providing improved aperture and gamma control, or in resetting CID imagers.